A method for controlling the load limit of a hydrostatic drive and a hydrostatic drive for a machine are described in EP 0 497 293 A1. It detects the position of the accelerator and the actual speed of the internal combustion engine present in the machine by means of metrological instruments and feeds these measured values to an electronic control system. A standard deviation is determined from the difference between the actual and target power levels defined by the measured values and a control valve actuated so that the hydraulic pump accepts hydraulic power lower than or equal to the available power of the internal combustion engine. The swivel angle of the hydraulic pump which varies as a function of the system pressure is not itself compensated, but only the change in the speed of the internal combustion engine thus arising, as an input variable for the control system is compensated.
This method of control displays a series of disadvantages. The control system can only allow for one load-dependent reduction in the speed of the internal combustion engine which has already taken place. In addition, the method described only allows for the pump for the hydraulic drive of the self-propelled machine. Other hydraulic loads are ignored in the calculation of the power data. Complicated load distributions and changes in them during operation, as commonly occur in complex hydraulic systems with multiple pumps and transmission systems, cannot be controlled satisfactorily by the method described.
The other load limit control systems known in the state of the art display similar shortcomings. The arrangement for operating a diesel-hydraulic transmission system known from DE 36 11 533 C1 uses a microprocessor controller to reduce the available hydraulic power in the event of thermal and/or mechanical overload of the diesel engine. However, it is necessary for the speed of the diesel engine to have fallen already so that the mechanical overload can be detected. In addition, if several adjustable pumps are present, their volume displacement will always be reduced uniformly, rendering flexible adjustment to different operating modes of the plant impossible.
The necessity for a so-called inching pedal constitutes a further disadvantage in modern machines. This may be separate or linked to the brake pedal and is used to increase the speed of the internal combustion engine independently of the speed of travel. In this way, the speed of the diesel engine can be increased when travelling slowly or standing still, in order to make additional power available to the pumps for further hydraulic functions, e.g. the lifting or working hydraulic systems. However, this complicates the operating procedure for the machine, as the operator has to ensure a diesel engine speed high enough to supply the respective hydraulic systems manually, by operating the inching and accelerator pedals, as well as operating the controls for the working functions.
The purpose of this invention is therefore to avoid the above-mentioned disadvantages and to provide a versatile, simple control method for self-propelled machines with several hydraulically-operated functions, the operation of which is simplified by comparison with systems currently common.
The invention achieves this by the method for controlling a hydraulic system, particularly of self-propelled machine with at least one internal combustion engine driving at least one hydraulic pump with adjustable volumetric displacement and possibly additional fixed-displacement pumps, described in claim 1, whereby:                the speed of the internal combustion engine is detected by a metrological instrument;        a the difference in pressure and the volumetric displacement of at least one hydraulic pump with adjustable volumetric displacement is determined by at least one measurement unit;        the power available from the internal combustion engine is determined from the speed measured;        the power consumed by each hydraulic pump with adjustable volumetric displacement is determined from the difference in pressure measured, the volumetric displacement and the speed;        so that the volumetric displacement of at least one hydraulic pump with adjustable volumetric displacement is controlled by a control system so that the total power consumed by at least one hydraulic pump with adjustable volumetric displacement is lower than or equal to the power available from the internal combustion engine or the power delivered or is restricted by the pump, if applicable, in the case of energy recovery at the hydraulic pump.        
The balance of power of the entire system can be determined with great accuracy because not only the speed of the diesel engine but also the difference in pressure and the volumetric displacement of the adjustable hydraulic pumps are measured. It is no longer necessary to detect the excess power delivered by a prior reduction in the speed of the diesel engine. On the contrary, the precise power consumption by each pump can be determined from the difference in pressure measured and the current volumetric displacement, and compared in the control device with the power known to be available from the internal combustion engine from the speed measured. The volumetric displacement of the adjustable pumps can thus be reduced before a reduction in the speed of the diesel engine in such a way that the total power consumed by the hydraulic pumps is always lower than or equal to the power delivered by the internal combustion engine. In this way, the engine can be prevented from stalling, even if there is a sudden increase in the load. An optimum internal combustion engine speed for the respective operating mode can be maintained, which improves the energy efficiency of the whole machine.
A further embodiment of the method is characterized in that the power consumed by each of the fixed-displacement pumps driven by the internal combustion engine is approximated from the speed of the drive by calculation and possibly the system pressure measured, and added to the total power consumed.
This renders it possible to integrate further fixed-displacement pumps driven by the internal combustion engine into the calculation of the hydraulic power delivered. Such fixed-displacement pumps are frequently present in common self-propelled machines, e.g. to operate the low-pressure system or for hydraulically-driven cooling fans, etc. Unlike the currently widespread failure to allow for these pumps, approximation by means of a speed-dependent value and allowance being made for it by the control system for the whole system response is advantageous. An even more precise estimate of the power consumed by calculation from the current system pressure produces a highly accurate balance of power at the drive train. This leads to safe operation of the machine in all its modes, as no hydraulic loads are ignored in the calculation of power,
It is advantageous if the calculation of the power of the internal combustion engine and/or the hydraulic pumps with adjustable volumetric displacement and/or the hydraulic fixed-displacement pumps takes place by means of stored effective relationships, particularly in the form of characteristic curves or families of characteristics. The driving torque accepted by the appropriate pump can be calculated accurately from the data measured, such as volumes displaced, differences in pressure, etc by means of previously-stored effective relationships. A balance of torque or power balance of the transmission system can be produced from the relationship between speed and the torque generated by the internal combustion engine. Allowance for changes in these effective relationships, e.g. due to symptoms of ageing or the replacement of individual components, can be made easily by appropriate changes to the control software.
If several hydraulic pumps with adjustable volumetric displacement are present, it is practical to set the volumetric displacement of the individual hydraulic pumps using stored control relationships, particularly for prioritizing individual hydraulic pumps. This allows the behavior of the machine to be adapted to a very wide range of use. In this way, the machine's hydraulic systems can be given priority over the transmission simply by adjusting the control relationships, whereby there is no need for the reduction to take place in all the pumps uniformly, but the working function can be given preference at the expense of the drive speed. This improves the overall behavior and ease of use of the system, and may enhance safety, as sufficient power for hydraulic circuits with a safety role can always be made available.
It may be advantageous for a control command from an operator to be detected by at least one input device, particularly an accelerator pedal and/or a joystick. In addition, if several hydraulic pumps with adjustable volumetric displacement are present, it may be advantageous for the volumetric displacement of these individual hydraulic pumps to be adjusted in an order of priority, allowing for the operators control commands.
Load distribution corresponding to the wishes of the operator may be obtained by allowing for the operator's control commands, such as, for example, the accelerator position. The power of the internal combustion engine can thus be fed to the transmission system for preference, if the accelerator pedal is strongly depressed. Analogously, the supply pumps of the machine's hydraulic systems can be taken into account to a greater extent than the other transmission systems and any necessary reduction in the power consumed be made in the other pumps in the case of high target settings for the machine's hydraulic system.
Another embodiment of the invention anticipates that the control system controls the power delivered or made available by the internal combustion engine by influencing its speed, in addition to adjusting the power consumed by the hydraulic pumps with adjustable volumetric displacement. In this way, operation of the machine can be controlled across wide ranges and the inching pedal may be waived. If the power made available by the internal combustion engine is insufficient for the calculated power delivered, the power of the internal combustion engine can automatically be increased to its maximum before the power consumed by the individual loads has to be reduced. This corresponds precisely to the function of the inching pedal, with which the operator affects this increase in power of the internal combustion engine manually, if he requires more power for a load. This allows the demands on the operator to be reduced and the productivity of the machine to be increased.
A further embodiment of the inventive method is characterized in that the power delivered to the internal combustion engine is integrated into the calculation of total power in operating modes in which a hydraulic pump with adjustable volumetric displacement acts as a drive (energy recovery from potential load and braking energy).
For example, when lowering loads or when the machine is travelling downhill, the respective displacement-controlled hydraulic transmission systems deliver power to the drive train through their pumps, which usually entails an increase in the speed of the internal combustion engine, for which the operator must compensate by decelerating. Such system modes may be detected by the control system introduced and be considered in the control of the entire system. This power delivered may then either be made available mechanically to another hydraulic load or lead to a reduction in the power provided by the internal combustion engine, which improves the energy efficiency of the overall system. In certain cases, more power can thus be made available to the hydraulic loads than is provided by the internal combustion engine installed in the machine.
It may be advantageous to allow for further measured system states, particularly vehicle speed, position of the machine's hydraulic system and the temperature of the hydraulic fluid, to control the individual hydraulic pumps with adjustable volumetric displacement.
The control system can be matched precisely to the current operating circumstances of the machine by allowing for such additional system static. In this way, the division of power between the individual pumps may be varied as a function of these static. For example, a corresponding prioritization of the transmission system may be achieved when traveling quickly or preference given to the machine's hydraulic system over the transmission system when executing working movements. The control system can also allow for additional hydraulic loads such as cooling fans, etc, depending upon the total power balance and the current temperatures.
A particular embodiment of the method is characterized in that in a case in which a hydrodynamic converter is provided for motive transmission, its power consumption, particularly from a stored speed-torque characteristic, will be calculated by the control system and taken into consideration in the total power calculation. The control system will also take into account if the machine is driven by a hydrodynamic converter instead of by a hydraulic motor with an adjustable pump (hydrostatic transmission). Calculation of the power consumed by the converter will then also take place using a family of characteristics which reflects the behavior of the converter. Allowance can thus be made for such power consumption in calculating the total output and the control system can apply the necessary signals to the appropriate control inputs of the converter transmission system to achieve the desired speed of travel.
The invention also concerns an electronic control system to implement the method according to one of the preceding claims. Such a control system may take the form of various embodiments in order to implement the method described above. Such systems usually consist of individual components, such as processor boards, memory boards, etc, which assume the individual control functions. The system data, the individual families of characteristics and power characteristics of the individual components can be changed by parameterization of the components and replaced if necessary, leading to a reduction in costs and improved efficiency of the whole system.